Distributed acoustic sensing (DAS) is a known technique wherein a length of optical fibre is interrogated, usually by one or more input pulses of light, to provide substantially continuous sensing of vibration activity along the length of the optical fibre.
Optical pulses are launched into the fibre and the radiation backscattered from within the fibre is detected and analysed. The intrinsic scattering from a continuous length of optical fibre is detected. Such sensors allow use of standard fibre optic cable without the need for deliberately introduced reflection sites such fibre Bragg gratings or the like. The entire optical fibre from which a backscatter signal can be detected can be used as part of the sensor. Time division techniques are typically used to divide the signal returns into a number of time bins, with the returns in each time bin corresponding to a different portion of the optical fibre. Such fibre optic sensors are referred to as distributed fibre optic sensors as the discrete sensing portions are fully distributed throughout the entire optical fibre.
Within each discrete sensing portion vibrations or strains of the fibre, for instance from acoustic sources, cause a variation in the characteristics of the radiation which is backscattered from that portion. This variation can be detected and analysed and used to give a measure of the disturbance of the fibre at that sensing portion. As used in this specification the term “distributed acoustic sensor” will be taken to mean a sensor comprising an optical fibre which is interrogated optically to provide a plurality of discrete acoustic sensing portions distributed longitudinally along the fibre and acoustic shall be taken to mean any type of mechanical vibration or pressure wave.
DAS therefore provides useful and convenient sensing solutions that can monitor long lengths of optical fibre with good spatial resolution. For instance a DAS sensor can be implemented with sensing portions of the order of 10 m long in up to 40 km or more of optical fibre, i.e. the 40 km length of fibre may be interrogated to provide 4000 contiguous acoustic sensing portions, each of 10 m in length.
Various types of DAS sensor are known and have been proposed for use in various applications. For instance such systems have been proposed for perimeter monitoring to detect the movement of land vehicles and personnel using a buried fibre optic cable. As a vehicle travels towards the buried fibre acoustic signals travel through the ground to the buried cable and the disturbance can be detected.
U.S. Pat. No. 5,194,847 describes a distributed acoustic fibre optic sensor for border monitoring and intrusion sensing. Coherent light is launched into an optical fibre and any light which is Rayleigh backscattered within the optical fibre is detected and analysed. A change in the backscattered light in a time bin is indicative of an acoustic or pressure wave incident on the relevant portion of optical fibre. In this way acoustic disturbances any portion of the fibre can be detected, which can be used as indication of an intruder. It is suggested that the optical fibre may be buried to detect foot or vehicular ground traffic or that an optical fibre placed above ground may be used to detect low-flying aircraft.
The ability to detect aircraft would be advantageous, especially in some border monitoring applications where other systems to detect aircraft crossing a border may not exists or would be unduly expensive.
GB patent application publication No. 2,442,745 describes a different fibre optic DAS system wherein a plurality of groups of pulse modulated electromagnetic waves are launched into a standard optical fibre. The frequency of one pulse within a group differs from the frequency of another pulse in the group. The Rayleigh backscattering of light from intrinsic reflection sites within the fibre is sampled and demodulated at the frequency difference between the pulses in a group.
Such DAS sensor systems can usefully be employed in perimeter monitoring to detect movement of personnel on the ground in the vicinity of the optical fibre, or to detect a vehicle crossing the optical fibre. However typically such systems simply indicate that a vehicle may have crossed at a particular location along the optical fibre. It would be useful to be able to provide more information about an object moving in the vicinity of the border. It is an object of the present invention to provide methods and apparatus for detecting moving objects using DAS.